Arc tubes typically are fabricated from tubular blanks. A quartz cylinder having an outer diameter of eight millimeters and a wall thickness of two millimeters is an example of such a blank. The bulbous midsection of the arc tube is formed in a portion of the blank, e.g., by means of a carbon die mold. During this step, the portions of the blank adjoining the midsection on both sides are not formed and retain their initial tubular shape. Electrode means are inserted into both ends of the blank such that the electrodes protrude into the interior of the midsection of the arc tube. Press seals are formed in the portions of the blank adjoining the midsection. In this step, the electrode means are fused within each press seal. An electric arc generating and sustaining fill is introduced into the interior of the bulbous midsection after which the midsection is hermetically sealed.
While this method generally has served the industry well, certain problems have arisen particularly in the manufacture of low-wattage miniature arc tubes where more precise manufacturing techniques are required. During the formation of the press seals, the opposed walls of the tubular blank are rapidly compressed and pinched together. The lateral flow of glass during the press operation may cause electrode misalignment or tearing of the molybdenum foil disposed in the press seal. The formation of the press seals may also deform the bulbous midsection; in a miniaturized lamp having precise arc tube geometry, such deformation may significantly detract from the operating characteristics of the arc tube.
It would constitute an advancement of the art if there were disclosed an improved method of fabricating an arc tube which would facilitate the manufacture of precision low-wattage miniaturized arc tubes.